The Eye of Harmony is the name given to the black hole that was captured by Rassilon and sealed at the center of the planet Gallifrey. It was created by the collapse of a black hole which was brought about by the Gallifreyian solar engineer Omega. The black hole itself was encased in a hexagonal, monolithic structure, and placed in the Panopticon which is the huge meeting place of the Time Lords. The Eye of Harmony can only be accessed with the Great Key or Rod of Rassilon. The Eye of Harmony is an almost inexhaustible source of energy which gives the Time Lords the ability to time travel and perform other such acts of temporal mechanics like time loops, a chronic hysteresis, and other such things. A remote Eye of Harmony is installed at the center of a TARDIS which is a link between a TARDIS and Gallifrey. The remote Eye of Harmony harnesses energy from the main Eye on Gallifrey and uses it travel. If that link were ever severed, a TARDIS would only be able to run on emergency power for a short period. After that the TARDIS would power down and be virtually dead.